[PATCH 2/2] ALSA: usb-audio: Do not expose sticky volume control mixers

[Rong Zhang]

Some devices expose sticky mixers that accept SET_CUR but do absolutely
nothing. Registering mixers for them confuses userspace and results in
ineffective volume control.

Check if the volume control is sticky by setting the volume to the
maximum or minimum value, and prevent the mixer from being registered
accordingly.

Quirky device sample:

  usb 7-1: New USB device found, idVendor=0e0b, idProduct=fa01, bcdDevice= 1.00
  usb 7-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
  usb 7-1: Product: Feaulle Rainbow
  usb 7-1: Manufacturer: Generic
  usb 7-1: SerialNumber: 20210726905926
  (Mic Capture Volume)

Signed-off-by: Rong Zhang <i@xxxxxxxx>
---
 sound/usb/mixer.c | 37 +++++++++++++++++++++++++++++++++++--
 1 file changed, 35 insertions(+), 2 deletions(-)

diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
index a25e8145af67..9f0aed36e27d 100644
--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -27,6 +27,7 @@
  *  	- parse available sample rates again when clock sources changed
  */
 
+#include <linux/array_size.h>
 #include <linux/bitops.h>
 #include <linux/init.h>
 #include <linux/list.h>
@@ -1287,17 +1288,49 @@ static int get_min_max_with_quirks(struct usb_mixer_elem_info *cval,
 		if (cval->res == 0)
 			cval->res = 1;
 
-		/* Additional checks for the proper resolution
+		/* Additional checks
+		 *
+		 * Some devices expose sticky mixers that accept SET_CUR
+		 * but do absolutely nothing.
 		 *
 		 * Some devices report smaller resolutions than actually
 		 * reacting.  They don't return errors but simply clip
 		 * to the lower aligned value.
 		 */
-		if (cval->min + cval->res < cval->max) {
+		if (cval->min < cval->max) {
+			int sticky_test_values[] = { cval->min, cval->max };
 			int last_valid_res = cval->res;
 			int saved, test, check;
+			bool effective = false;
+
 			if (get_cur_mix_raw(cval, minchn, &saved) < 0)
 				goto no_res_check;
+
+			for (i = 0; i < ARRAY_SIZE(sticky_test_values); i++) {
+				test = sticky_test_values[i];
+				if (test == saved)
+					continue;
+				/* Assume non-sticky on failure. */
+				if (snd_usb_set_cur_mix_value(cval, minchn, 0, test) ||
+				    get_cur_mix_raw(cval, minchn, &check) ||
+				    check != saved) { /* SET_CUR effective, non-sticky. */
+					effective = true;
+					break;
+				}
+			}
+			if (!effective) {
+				usb_audio_warn(cval->head.mixer->chip,
+					"%d:%d: sticky mixer values (%d/%d/%d => %d), disabling\n",
+					cval->head.id, mixer_ctrl_intf(cval->head.mixer),
+					cval->min, cval->max, cval->res, saved);
+
+				cval->initialized = 1;
+				cval->min = cval->max = cval->res = 0;
+				return -ENODEV;
+			}
+
+			if (cval->min + cval->res >= cval->max)
+				goto no_res_check;
 			for (;;) {
 				test = saved;
 				if (test < cval->max)

-- 
2.53.0